This project is to identify systemically active peptidomimetics capable of attenuating morphine tolerance and withdrawal syndrome. Tolerance and withdrawal in rats is associated with peptides with -Arg-Phe-NH/2 C- termini, (eg NPFF and FMRF-NH/2), and the tetrapeptide that has been shown to bind to the NPFF receptor with highest affinity is FFRF-NH/2. Peptidomimetics of these substances will be prepared and tested to evaluate and develop pharmacological interventions which modulate opiate tolerance, dependence, and abstinence. Evaluation of the compounds will involve four stages. First, their relative stabilities to proteolytic degradation will be determined. Binding constants then will be measured for the affinity of these compounds to the NPFF receptors. Compounds with high receptor binding affinities and good proteolytic stability, will be administered subcutaneously to rats to gauge their anti-opiate agonist or antagonist properties. Finally, the most active of these substances also will be tested for permeation across the blood brain barrier (BBB). The following simple peptides will be prepared and tested to reveal the effects of changes near the N-terminus: dansyl-X/1FRF-NH/2, dansyl-X/1FR- NH2, daFFRF-NH/2, daFFR-NH/2, FLFQX/1FRF-NH/2, and approximately 40 combinations of XXXXFFRF-NH/2 (throughout X/1 = F or P,X = a range of D and L amino acids representing hydrophobic, hydrophilic, acid, basic, and neutral residues, Table 2). In this way the influence of stereoelectronic changes, N-dansylation, and incorporation of desaminophenylalanine (daF) will be probed. Simple changes at the C-terminus have already been explored extensively, thus paving the way for more advanced studies. Consequently, over 40 FFRF-NH/2 peptidomimetics incorporating sterically constrained surrogates of protein amino acids also will be prepared (Table 3), with particular emphasis on modifications of the RF-NH/2 "warhead" region (which is critical for receptor binding). Specifically, the protein amino acid analogs to be used are alpha-methylarginine, alpha- methylphenylalanine, alpha-methylmethionine, 1,2,3,4- tetrahydroisoquinoline-3-carboxylic acid (Tic), and 2,3-methanoamino acid surrogates for Met, Arg, and Phe. Peptidomimetics formed from these compounds are intended to be constrained into various molecular orientations, some of which may resemble the bioactive conformation of the parent peptide as it binds to the NPFF receptor. Incorporation of suitable rigid amino acid analogs, or combinations of these, therefore could enhance receptor binding by decreasing the entropy gains associated with the docking process.1 They will also be relatively bio-available because of their high proteolytic stabilities. In summary, the goal of this work is to identify strong binding NPFF agonists and antagonists which are relatively stable to proteolytic degradation, and which can be administered subcutaneously.